docs: def _handle_external_link(self, token):
          """Handle when an external link is at the head of the tokens."""
          brackets, url = token.brackets, None
          self._push()
          while self._tokens:
              token = self._tokens.pop()
              if isinstance(token, tokens.ExternalLinkSeparator):
                  url = self._pop()
                  self._push()
              elif isinstance(token, tokens.ExternalLinkClose):
                  if url is not None:
                      return ExternalLink(url, self._pop(), brackets)
                  return ExternalLink(self._pop(), brackets=brackets)
              else:
                  self._write(self._handle_token(token))
          raise ParserError("_handle_external_link() missed a close token")